An optical disc apparatus has a structure in which accurate data recording and reproduction are enabled by the performance of focus error correction control to focus on the information layer, tracking control to prevent deviation from information tracks formed in the information recording layer of the optical disc, and tilt control to control the optical axis of the laser beam output from the objective lens so that it is perpendicular to the surface of the recording medium.
At present, various types of optical discs have been developed, including the DVD (digital versatile disc) and BD (Blu-ray disc) for use in TV program recording and reproduction and PC applications. The shorter laser beam wavelengths and larger NA used to obtain higher densities increase the effect of comatic aberration, demanding more precise tilt control.
As one proposed answer to this demand, in tilt control of an optical disc inserted into an optical disc apparatus, a tilt adjustment is performed at a radial position midway between the inner circumference position and the outer circumference position (see, for example, patent document 1). Another proposal is to perform tilt adjustments at a plurality of radial positions, and perform corrections with estimated tilt values obtained from an approximation formula calculated from the adjustment values (see, for example, patent document 2).    Patent document 1: Japanese Patent Application Publication No. 2003-263764 (page 5, FIG. 2)    Patent document 2: Japanese Patent Application Publication No. 2004-055103 (page 17, FIG. 10)